This invention relates generally to the fields of immunology and medicine and more specifically to methods for generating an immune response against HIV-1.
The incidence of acquired immunodeficiency syndrome (AIDS) has reached epidemic proportions, particularly in third world countries in Africa, Asia and the Caribbean. The causative agent of AIDS is the human immunodeficiency virus (HIV-1). HIV-1 is a primate lentivirus believed to have crossed species to humans from a monkey population in Africa in the 1950""s.
In humans, HIV-1 infects two primary cell types: macrophages and T lymphocytes. In a typical transmission event, a macrophage-tropic strain is passed from a bodily fluid of one person to macrophages in the skin or mucosa of another. Initially, the virus can replicate in macrophages without producing clinical disease. As the infection progresses, divergent strains are produced as a result of the virus""s rapid mutation rate, due at least in part to a reverse transcriptase having poor fidelity. Eventually, T cell-tropic strains arise which result in depletion of the immune system and the clinical manifestations known as AIDS.
This progression typically takes from six to ten years or more, depending, in part, on host immune system factors. However, a T-cell-tropic HIV-1 strain can also be transmitted directly leading to a much more rapid course of infection. As the disease progresses, the host""s immune system is unable to replace the T cell population as quickly as it is depleted by viral infection, leading to catastrophic decreases in T cell levels. As a result, AIDS patients typically die from infections that normally would cause, at worst, a mild illness in a healthy individual.
Despite the expenditure of billions of dollars for research, only moderate progress has been made in treating the disease. Initially, therapy was limited to methods of treating the opportunistic infections which arose in the immunocompromised patients. Later, nucleoside analogs such as AZT, ddC and ddI were identified that could interfere with HIV-1 replication. These drugs, particularly in combination, prolonged the lives of some AIDS patients. More recently, a class of drugs called protease inhibitors has proven to be even more effective in inhibiting HIV-1 replication. When used in combination with nucleoside analogs, protease inhibitors have reduced viral titers in some patients to undetectable levels. Recently, the death rate from AIDS in the United States has decreased for the first time since the start of the epidemic. This is believed to be a result of such treatment regimens, at least in part, along with educational programs discouraging high risk behaviors.
Hopes for these treatment regimens have been curtailed by the subsequent emergence of resistance to multidrug cocktails. Additionally, while circulating levels of virus drops in some patients treated with combinations of protease inhibitors and nucleoside analogs, such treatment does not result in elimination of the virus from those patients. Viral reservoirs are believed to remain even in patients having no detectable circulating virus, for example in the lymphatic system and in macrophages. Furthermore, even where no circulating virus is present, viral spread may still occur as a result of the fusogenic properties of cells already infected with HIV-1. Finally, the drug cocktails have not produced positive results in all patients, and therefore appear unlikely to be effective against all viral strains.
The cumbersome dosing regimen required by multidrug therapies also creates problems with patient compliance. As treated patients survive for longer periods of time, these compliance issues will only increase, leading to the possibility that such patients may eventually present avenues for the generation and spread of HIV-1 strains resistant to the drug cocktails.
Furthermore, although some patients in the U.S. can afford such expensive combination therapies, the cost of such drugs far exceeds the annual income of many third world individuals. This is particularly alarming because of the rapid increase in HIV-1 transmission in the third world.
Clearly, a preferable approach would be to prevent HIV-1 transmission. Vaccines are a logical choice to accomplish this. For example, vaccines have been used to prevent or reduce the severity of various viral diseases, including polio, measles, smallpox and influenza. In addition, a vaccine can stimulate the immune system in individuals already infected with a virus. For example, rabies vaccine is only administered after a transmission event, such as a bite from an infected animal.
Numerous attempts have been made to develop a vaccine that would increase a person""s resistance to HIV-1 infection, but the approaches to date suffer from serious limitations. For example, vaccines composed of portions of an HIV-1 protein or using a killed HIV-1 virus have been produced. However, HIV-1 mutates rapidly during viral replication, producing new variants faster than the immune system can react. As a result, an immune response which is stimulated against a particular HIV-1 protein or strain can be ineffective against variants that arise during infection. Furthermore, vaccination with portions of HIV-1 surface proteins can stimulate the production of antibodies that may facilitate, rather than prevent, HIV-1 infection. Attenuated vaccines, which consist of live but reproductively defective viruses, also have been proposed. However, there is justified concern for injecting such an HIV-1 virus into an individual, particularly an otherwise healthy person. Thus, a need exists for a vaccine that provides an immune response against HIV-1 but does not carry the attendant risks and limitations associated with the use of HIV-1 as the vaccinating agent.
One approach to developing an animal model to study HIV-1 infection and possible vaccine strategies has been the generation of a recombinant virus between simian immunodeficiency virus (SIV) and HIV-1, or SHIV, which can infect lower primates. The in vivo behavior of the SHIV model has elucidated many aspects of retroviral pathology and host response. These include the demonstration that host range, cytopathicity and the specificity of neutralizing antibodies are determined by the envelope sequences (Luciw et al., Proc. Natl. Acad. Sci. 92:7490-7494 (1995)). Used as live-attenuated vaccines, SHIVs have been shown to effectively protect macaques against mucosal challenge with SIV (Baba et al., Science 267:1820-1825 (1995); Quesada-Rolander et al., AIDS Res. Hum. Retro. 12:993-999 (1996)).
In 1985 and 1986, the relationship between the nucleotide and amino acid sequences of HIV-1 and caprine arthritis-encephalitis virus (CAEV) was described. CAEV and HIV-1 are closely related phylogenetically and share a high degree of homology, including, for example, between their RNA-dependent DNA polymerases (pol) and between gp120/41 in HIV-1 and gp135/38 in CAEV (see, for example, Gonda et al., Proc. Natl. Acad. Sci., USA 83:4007-4111 (1986); Gonda et al., Retroviridae 3:83-109 (1994); Garry et al., Retroviridae 4:491-603 (1995)).
CAEV infects goats and causes abnormalities of the immune system in some infected animals (see, for example, Banks et al., Arthrit. Rheum. 30:1046-1053 (1987); Crawford et al., Science 207:997-999 (1980)). CAEV is associated with three disease syndromes in goats: arthritis, which occurs in 20-30% of infected animals; leukoencephalitis, which occurs in young animals; and sporadic neurologic disease, which occurs in adult goats. However, 60% of infected animals are long-term non-progressors, developing no clinically apparent lesions (Cheevers et al., Lab. Invest. 58:510-517 (1988); Knowles et al., J. Virol. 64:2396- 2398 (1990); Perry et al., J. Infect. Dis. 171:328-334 (1995)). CAEV infection is found worldwide, with an estimated 80% of goat herds suffering from some level of infection.
CAEV is transmitted among goats through infected milk, particularly colostrum, and infection is spread by the agricultural practice of pooling colostrum to feed young animals. CAEV multiplies in cells of the monocyte/macrophage lineage and in fibroblast cell lines, but does not infect T cells. Macrophages expressing CAEV are distributed in the synovia, lungs, central nervous system, lymph nodes, spleen, gastro-intestinal tract and mammary glands of infected goats.
There is a need in the art for a non-pathogenic lentivirus which can generate an immune response against HIV-1 in a primate, particularly a human.
The present invention satisfies this need and provides additional advantages. In one embodiment, a polynucleotide comprising a chimeric retroviral genome is provided comprising elements of CAEV and HIV-1. This chimeric retrovirus is referred to as a CHIV. The retroviral genome includes the regulatory sequences from CAEV along with such other CAEV coding sequences as are required to render the CHIV nonpathogenic, as well as an HIV-1 env gene. Other retroviral genes can be provided from either CAEV or HIV-1. The CHIV must be able to propagate in at least one mammalian host cell.
The invention further provides a CHIV immunogen capable of stimulating an immune response. In one embodiment, the CHIV immunogen is a viral particle capable of infecting a host cell. In another embodiment, the CHIV immunogen is a polynucleotide expression construct which can be expressed when introduced into an organism in order to stimulate an immune response to the expression products.
The present invention also provides a vaccine comprising a CHIV immunogen and a pharmaceutically acceptable carrier. A vaccine of the invention is useful, for example, to stimulate an immune response against human immunodeficiency virus (HIV-1) in a human or other primate.
The invention also provides a method of stimulating an immune response in an individual against HIV-1 by administering a CHIV immunogen to the individual. Such a method is useful, for example, to increase the resistance to HIV-1 infection of an individual not previously exposed to HIV-1, or to reduce the severity of a pathology caused by HIV-1 in an HIV-1 infected individual. In addition, the invention provides a method of stimulating an immune response in vitro by contacting a lymphocyte with a CHIV immunogen.